Was Lancelot the greatest Knight of them all? Explorations in Arthurian History>>
Lancelot is entirely the creation of Chretien de Troyes and is as such no
part of historical investigation. Some scholars have tried to tie the story
of Lancelot to various companions and/or opponents of Arthur.

Explorations in Arthurian Legend>>
The answer is yes, according to Chretien de Troyes. He it was who
invented Lancelot and added him to Arthur's court as a Knight of the Round
Table. In his four romances, Chretien portrays Lancelot as the best of all
the knights: skilled in arms and chivalry and courtly love and all the other
qualities that make a knight successful.

He is the king's champion, fighting challengers and going on quests in
the king's name. He is time and again the symbol of Arthur's justice.
Those who question the queen's fidelity are made to pay by fighting Lancelot.
(Ironic, isn't it?) He is the most famous of the knights and the most
accomplished. And yet, he is not the most perfect. That honor falls to
his son, Galahad.

It is interesting to note here that Lancelot is vilified by some for carrying
on an adulterous affair with Guinevere while at the same time glorified
by others for begetting Galahad, the perfect knight, from a tryst with Elaine,
to whom he was not married. (Some stories say they did marry, but the begetting
was before that.) And just as Lancelot had supplanted Gawain in being the
best knight-at-arms, so does Galahad supplant Lancelot in being the best
knight-in-spirit. As the legend writers turned to the Holy Grail, they incorporated
the idea of purity of body and mind; Lancelot did not fill the bill.

His son saw the Grail and realized the Grail Quest; the father was reduced
to seeing an obscured vision and dying knowing that his own sin caused his
inability to see the wondrous object. Lancelot was the best at arms but
not in piety.